The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to, a method and apparatus for rapid whole body scanning with continuous table motion and image acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Conventional techniques for whole body imaging or screening using magnetic resonance imaging typically requires multiple positioning and repositioning of the patient to image over a sufficiently large field-of-view (FOV). As a result, whole body screening examinations are often partitioned into two or more separate examinations. In many instances, a patient must return at a later date to complete the examination due to contrast agent uptake and passthrough.
Other so-called moving table techniques include stepping the table through increments and obtaining data at each increment. These techniques do not acquire data while the table is moving, but only between each of the stepped increments. These techniques are not time-efficient and since the increments must be pre-defined and the acquisitions times accordingly, there is no ability to provide interactive control and the results are prone to gradient warping. Further, attempts at imaging while the table is moving are also prone to gradient warping distortion and require some form of image correction. Such techniques may also require phase encoding in the direction of table motion and require predefined knowledge of table speed and/or acceleration/deceleration and are therefore not susceptible to interactive control during image acquisition.
It would therefore be advantageous to design a method and apparatus incorporating a fast technique that is sensitive to abnormalities and allows the physician to quickly survey the entire body to locate regions of abnormalities, such as signal enhancement that is indicative of tumors, with a continuous moving table and with the ability to change the acquisition plane, the pulse sequence, table speed and/or the direction of table motion to thereby focus in and better characterize the abnormality.